Earth’s energy balance is unbalanced

You probably remember your elementary science teachers explaining that energy cannot be created or destroyed. It is a fundamental property of the universe.

Energy can however be transformed. When the Sun’s rays hit Earth, they turn into random motions of molecules that you feel as heat. At the same time, the Earth and the atmosphere send radiation back into space. The balance between incoming and outgoing energy is known as the Earth’s “energy budget”.

Our climate is determined by these energy flows. When the amount of incoming energy is greater than the outgoing energy, the planet heats up.

This can happen in a number of ways, for example when sea ice that normally reflects solar radiation back into space disappears and the dark ocean absorbs that energy instead. It also happens when greenhouse gases build up in the atmosphere and trap some of the energy that would otherwise have been irradiated.

Scientists like me have been measuring the Earth’s energy balance since the 1980s using instruments on satellites, in the air and oceans, and on the ground. Find out more about these measures and Earth’s energy budget when the report of the United Nations Intergovernmental Panel on Climate Change is released on August 9.

Here’s a closer look at how energy flows and what the energy balance tells us about how and why the planet is warming.

Balancing the energy of the Sun

Virtually all of the energy in Earth’s climate system comes from the Sun. Only a tiny fraction is driven upwards from the interior of the Earth.

On average, the planet receives 340.4 watts of sun per square meter. All of the sun is falling on the day side and the numbers are much higher at local noon.

Of those 340.4 watts per square meter, 99.9 watts are reflected back into space by clouds, dust, snow and the earth’s surface.

The remaining 240.5 watts are absorbed – about a quarter by the atmosphere and the rest by the surface of the planet. This radiation is transformed into thermal energy within the Earth system. Almost all of this absorbed energy corresponds to the energy returned to space. A tiny residue – 0.6 watts per square meter – accumulates as global warming. It may not seem like much, but it adds up.

The atmosphere absorbs a lot of energy and emits it as radiation both in space and towards the surface of the planet. In fact, the Earth’s surface receives almost twice as much radiation from the atmosphere as from the direct sun. This is mainly because the Sun only heats the surface during the day, while the warm atmosphere is up there 24/7.

Together, the energy reaching the Earth’s surface from the Sun and the atmosphere is approximately 504 watts per square meter. The Earth’s surface emits about 79% of this return. The remaining surface energy is used to evaporate water and heat the air, oceans and land.

The tiny residue between the incoming sun and the outgoing infrared is due to the build-up of greenhouse gases like carbon dioxide in the air. These gases are transparent to sunlight but opaque to infrared rays – they absorb and emit a lot of infrared rays.

The Earth’s surface temperature must increase in response until the balance between incoming and outgoing radiation is restored.

What does this mean for global temperatures?

Doubling carbon dioxide would add 3.7 watts of heat to every square meter of the Earth. Imagine old-fashioned incandescent nightlights spaced every 3 feet across the globe, left on forever.

At the current rate of emissions, greenhouse gas levels would double from pre-industrial levels by mid-century.

Climate scientists calculate that adding so much heat to the world would warm the Earth’s climate by about 5 degrees Fahrenheit (3 C). To avoid this, the burning of fossil fuels, the main source of greenhouse gas emissions, should be replaced by other forms of energy.

This article is republished from The Conversation under a Creative Commons license. Read the original article here: https://theconversation.com/earths-energy-budget-is-out-of-balance-heres-how-its-warming-the-climate-165244.

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